1. T.H. Morgan – 1910 –Working with fruit fly, Drosophila melanogaster –Discovered a mutant male fly with white eyes instead of red –Crossed the mutant male to a normal red-eyed female All F 1 progeny red eyed = dominant trait

2. Morgan crossed F 1 females x F 1 males F 2 generation contained red and white- eyed flies –But all white-eyed flies were male Testcross of a F 1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene resides on the X chromosome Morgan crossed F 1 females x F 1 males F 2 generation contained red and white- eyed flies –But all white-eyed flies were male Testcross of a F 1 female with a white-eyed male showed the viability of white-eyed females Morgan concluded that the eye color gene resides on the X chromosome

3. Sex Chromosomes Sex determination in Drosophila is based on the number of X chromosomes –2 X chromosomes = female –1 X and 1 Y chromosome = male Sex determination in humans is based on the presence of a Y chromosome –2 X chromosomes = female –Having a Y chromosome (XY) = male

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6. Sex-Linked Inherited Traits In many organisms, the Y chromosome is greatly reduced or inactive. genes on the X chromosome are present in only 1 copy in males Sex-linked traits: controlled by genes present on the X chromosome Certain genetic diseases affect males to a greater degree than females –X-linked recessive alleles Red-green color blindness Hemophilia

7. Humans have 46 total chromosomes –22 pairs are autosomes –1 pair of sex chromosomes –Y chromosome highly condensed Recessive alleles on male’s X have no active counterpart on Y –“Default” for humans is female Requires SRY gene on Y for “maleness”

8. Hemophilia Disease that affects a single protein in a cascade of proteins involved in the formation of blood clots Form of hemophilia is caused by an X-linked recessive allele –heterozygous females are asymptomatic carriers Allele for hemophilia was introduced into a number of different European royal families by Queen Victoria of England

10. Dosage Compensation Ensures an equal expression of genes from the sex chromosomes even though females have 2 X chromosomes and males have only 1 In each cell of a female, 1 X chromosome is inactivated and is highly condensed into a Barr body –the other X chromosome provides phenotype Females heterozygous for genes on the X chromosome are genetic mosaics

12. Genetic Mapping Early geneticists realized that they could obtain information about the distance between genes on a chromosome Based on genetic recombination (crossing over) between genes If crossover occurs, parental alleles are recombined producing recombinant gametes

14. Alfred Sturtevant –Undergraduate in T.H. Morgan’s lab –Put Morgan’s observation that recombinant progeny reflected relevant location of genes in quantitative terms –As physical distance on a chromosome increases, so does the probability of recombination (crossover) occurring between the gene loci

15. Constructing Maps The distance between genes is proportional to the frequency of recombination events recombination frequency recombinant progeny total progeny 1% recombination = 1 map unit (m.u.) 1 map unit = 1 centimorgan (cM) =